This present invention relates generally to sorting of waste and recyclable materials using optical techniques. More specifically, this invention relates to the sorting of glass cullet and similar items by measuring the attenuation of light radiated through the cullet.
In conjunction with a continuing worldwide need to preserve natural resources and reduce dependence on landfills and similar waste storage facilities, machines and methods have been devised for automated identification and sorting of waste materials. Among the waste materials of interest is glass cullet, e.g., small pieces of glass of varying characteristics that are distinguished by color. For example, a typical collection of glass cullet may include pieces of glass having green, red, and blue color components or combinations thereof Prior art glass sorting machines function by sliding the pieces of cullet down what is commonly called a xe2x80x9cwearcover.xe2x80x9d At one or more locations along the wearcover, the cullet will slide under one or more light sources and over one or more light sensors or light receivers arranged to define a sensing area. The pieces of glass cutlet having different color characteristics will attenuate the light emitted from the light sources in different amounts. For instance, if a piece of red glass passes between red and green light sources and the light sensors, the green light, as measured by the light sensors, will be attenuated more than the red light.
Most prior art glass sorters have, in fact, employed optical techniques relying primarily on red and green LED light sources. The primary reason for this is that red and green LED""s and sensors were the only colors readily available at economically feasible prices. Unfortunately, the use of only red and green light sources in glass sorting restricts the ability of the machine to accurately identify glass containing other color components. This has resulted in the inability of prior art glass sorter to reliably distinguish cullet having measurable level of a blue component. xe2x80x9cBluesxe2x80x9d (cullet containing a measurable level of a blue color component) are either discarded as waste along with other non-distinguishable impurities, or were mis-sorted in with another color if the xe2x80x9cbluexe2x80x9d contained a relatively small blue color component. This mis-sorting results in a less pure, lower quality sort, which leads to a lower quality recycled product. Thus, the economic value of the sorted lot, as well as the quality the final product, is lower due to mis-sorts.
Another drawback with prior art glass sorting machines is that a film of crud or other impurities slowly builds up on the wearcover, thereby blocking the light sensors. This buildup, overtime, attenuates all light wavelengths to a measurable level. The film buildup is a by-product of the dirt, sand, water and other material that the cullet sit in, or are exposed to, prior to sorting. Since the cullet are generally trash to be recycled, it is, generally, not cost effective to pre-clean the cullet.
Edge refraction and impurity adhesion also cause glass mis-sorts. Cullet are generally relatively small broken pieces of glass with edges facing a variety of different directions. Light is refracted off at different angles from the different edge angles. The edges also create a prism effect. Because the light is re-directed at different angles, the edges will appear opaque to a sensor. Since cutlet are typically one-half inch to two and one-half inch across with edges typically one-sixteenth to one-quarter inch deep, the amount of light refracted, relative to the amount of light passing through the cullet, is not insignificant. This can lead to incorrect color selection and sorting of the cullet or rejection of cullet as foreign matter.
A further problem associated with film buildup on the wearcover of a prior art glass sorter is a shoveling or dozing effect created by cullet in the built-up film layer. This shoveling leaves furrows and other non-uniformities in the film layer. Prior art machines have attempted to compensate for this by slowly adjusting or re-normalizing a baseline light sensor reading, or amplitude value, over time. This has been less than satisfactory due to the non-uniformities previously discussed. Other prior art glass sorting techniques have tried to address this problem by attempting to clean the wearcover and by replacing the wearcover when the film build-up is excessive. However, cleansers have been ineffective and often leave a residue. Frequent replacement of the wearcover is expensive and leads to excessive down time of the sorting unit.
Thus, to increase the economic viability of recycling in this era of limited resources, more accurate glass sorting machines are needed. In the areas where landfill space is still relatively cheap, reducing recycle costs is perhaps even more important since the need for a landfill alternative is not as great. Additionally, increasing the quality of the final sorted product and reducing the product""s cost will help shift the market from virgin raw material to post consumer material. This will reduce the need to consume the earth""s limited resources. What is needed, then, is an efficient and economical method for sorting cullet by color, including the ability to distinguish blue color components. A means to compensate for non-uniform film build up over the light sensors is also needed, as is an ability to detect and correct errors due to edge refraction. An ability to distinguish ceramics from other opaque objects would be useful. Another useful capability would be to distinguish glass transparent in the visible spectrum from glass opaque in the infrared spectrum. Such xe2x80x9cIRxe2x80x94opaquexe2x80x9d glass has a different economic value than xe2x80x9cIRxe2x80x94transparentxe2x80x9d glass.
This invention relates to optical sorting of glass cullet by use of red, green and blue LED light sources, and, optionally, an infrared light source. This invention is capable of accurately identifying and sorting glass cullet based on colors having a blue component or based on yellow coloration resulting from a lack of a blue color component. Accordingly, the invention can distinguish glass colors that prior art machines could not, including purple, violet, cyan, teal, amber, yellow, and blue.
In one embodiment of the invention, glass cullet are passed between light emitting diodes and light sensors arranged to define a light sensing region. Each LED emits light of a red, green or blue wavelength. Cullet of one color will attenuate light wavelengths corresponding to other colors by different amounts than a wavelength corresponding to the same color as the cullet. By comparing the attenuation amounts of the color components, red, green, and blue, with color component intensity values of the known colors, the cullet color can be identified.
In another embodiment, a pixel grid bit map of the light sensing region is maintained in a microprocessor. The pixel grid should be sufficiently small so as to result in a substantially constant sensed light magnitude across the grid and be large enough to keep computation time to an acceptable duration. A grid size of approximately xe2x85x9 inch square is nominally acceptable.
Each grid typically receives a red, green, and blue digital signal. A digital signal is sent from an analog-to-digital converter (A/D) which receives the analog signal from the light sensor. The light sensors have a baseline value, or amplitude, which corresponds to the magnitude of light received by the sensor when no cullet are in the light path to attenuate the light. Attenuated signals are compared against these baseline values to determine the amount of attenuation for given cullet. Thus, a pixel grid bitmap of measured color amplitudes of the cullet is generated. A microprocessor performs the color analysis, or comparison, to determine the color of the cullet. The cutlet continues down stream in the sorter to be ejected if its color matches a color chosen for ejection.
The problem of misreading colors due to film buildup, non-uniformities caused by cullet furrowing, and other non-uniformities is overcome by an embodiment of this invention. To reduce these problems, the invention re-normalizes the baseline value of the signal of the sensed light when cullet are not in the sensing region, and thus attenuating light. Prior art sorting methods simply increasedxe2x80x94or decreased depending on the computation algorithms usedxe2x80x94the baseline value over time. This was not fully successful due to the non-uniformities in the film layer.
One method to determine when cullet are absent from the sensing region is to take a derivative of the sensed signal. If the signal is changing, there is cullet in the sensing region and the derivative will have a value other than zero. When there is no cullet is in the sensing region, the derivative will be zero, even if the magnitude of the sensed signal has changed. When no cutlet are present, the baseline value of the signal may be re-normalized.
This will compensate for film buildup, sudden furrows, and similar non-uniformities on the wearcover in the sensing region. A typical microprocessor can operate to take the derivative and re-normalize the system. However, a discrete signal processor (DSP) is more effective. A DSP is a specialized microprocessor that is optimized for high speed numerical processing. A DSP will operate quickly enough to re-normalize the system between times that cullet are present in the sensing region even though the system is operating at a high rate of speed.
The pixel grid bitmap may also be used to compensate for edge refraction and impurity adhesion. An edge of a cullet will generally appear opaque due to edge refraction and prism effects. The cullet color will often not be accurately identified because of the opaque or darker edges. A data erosion technique using a threshold ejection footprint (or erosion footprint) is used to compensate for refraction. The footprint, which is typically adjustable, corresponds to the required number of contiguous pixel grids below which the sensed signal is ignored or suppressed. Imbedded impurities can be compensated for using the same technique.
Another embodiment of the invention employs infrared light sources and sensors capable of detecting infrared wavelengths in addition to, or independent of, the red, green, and blue light sources. The infrared wavelength is particularly useful for distinguishing cullet with paper labels from ceramics and bottle caps, and the like. IR light is better than visible (R G B) light at penetrating paper labels that may be attached to the cullet. This prevents glass with a paper label from being misidentified as opaque (ceramic).
There is a difference between the economic value of infrared opaque glass and infrared transparent glass. The difference in value can be significant. An IR light source could be used to distinguish these two types of glass.
Including infrared sources and sensors in the sensing region is quite useful and relatively simple. The red and infrared sources emit stronger signals than the green sources. The infrared sources can be retrofitted into the light source (or light array) relatively easily by alternating red and infrared sources in one row of previously all red sources. Generally intermixing the red and infrared sources is equally effective. These IR-R rows would likely be used with two rows of green sources and one row of blue sources. Again, all the sources may be intermixed and need not be separated into rows.
Use of a collimator is recommended. The collimator reduces the perceived angle of light and enhances shadow definition. The collimator generally abuts the light sources, thereby restraining the emitted light into narrow beams or channels. These narrow beams of emitted light are then interfered with and attenuated by the cullet as they pass through the beam.
An object of this invention is to provide an efficient and economical method for sorting cullet by color, including the ability to distinguish blue color components. Another object is to provide a means to compensate for non-uniform film build up over the light sensors. Yet another object is to provide an ability to detect and correct errors due to edge refraction. An ability to distinguish ceramics from other opaque objects is another object of the invention. Yet another objective is to distinguish between visibly transparent glass which is opaque in the infrared spectrum from visibly transparent glass which is transparent in the infrared spectrum.